Methods and apparatus to bias spool valves using supply pressure

ABSTRACT

Methods and apparatus to bias spool valves using supply pressure are disclosed. An example apparatus includes a housing of a spool valve, the housing including a first port to receive a fluid at a supply pressure. The example apparatus further includes a spool within the housing. A position of the spool is to be selectively controlled via an input force acting on the spool, the position of the spool to define a path of a flow of the fluid through the spool valve from the first port to a second port of the housing. A biasing force is to bias the spool opposite the input force, the biasing force to be generated from the supply pressure applied to an end of the spool.

FIELD OF THE DISCLOSURE

This disclosure relates generally to spool valves and, moreparticularly, to methods and apparatus to bias spool valves using supplypressure.

BACKGROUND

Spool valves are a common component in many hydraulic and/or pneumaticmachines and systems. Spool valves are used to control and/or direct theflow of fluid along different paths between one or more input ports ofthe spool valve to one or more output ports based upon the position of aspool within the spool valve.

SUMMARY

Methods and apparatus to bias spool valves using supply pressure aredisclosed. An example apparatus includes a housing of a spool valve, thehousing including a first port to receive a fluid at a supply pressure.The example apparatus further includes a spool within the housing. Aposition of the spool is to be selectively controlled via an input forceacting on the spool, the position of the spool to define a path of aflow of the fluid through the spool valve from the first port to asecond port of the housing. A biasing force is to bias the spoolopposite the input force, the biasing force to be generated from thesupply pressure applied to an end of the spool.

Another disclosed example apparatus includes a spool within a spoolvalve to control a flow of fluid at a supply pressure through the spoolvalve as the spool is moved via an input force. The spool valve is todefine a chamber adjacent an end of the spool. A biasing force is tobias the spool opposite the input force, the biasing force to begenerated from a biasing pressure of the fluid within the chamber. Thebiasing pressure is to be based on the supply pressure.

Another disclosed example apparatus includes a spool within a spoolvalve, the spool to be selectively moveable within the spool valve viaan input force to control a flow of a supply fluid between ports in thespool valve, the supply fluid having a supply pressure. The exampleapparatus further including means for generating a biasing force to biasthe spool opposite the input force, the biasing force corresponding to abiasing pressure applied to an end of the spool. The biasing pressure tobe based on the supply pressure of the supply fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are cut-away perspective views of an example spool valveconstructed in accordance with the teachings disclosed herein with thespool in different positions.

FIG. 4 is a cross-sectional view of the example spool valve of FIGS.1-3.

FIG. 5 is a partially cut-away exploded view of the example spool valveof FIGS. 1-4.

FIG. 6 is a cross-sectional view of another example spool valveconstructed in accordance with the teachings disclosed herein.

FIG. 7 is a partially cut-away exploded view of the example spool valveof FIG. 6.

FIG. 8 is a cut-away view of another example spool valve constructed inaccordance with the teachings disclosed herein.

FIG. 9 is a cross-sectional view of another example spool valveconstructed in accordance with the teachings disclosed herein.

FIG. 10 is a schematic drawing of an example digital valve controllerthat includes the example spool valve of FIGS. 1-5.

DETAILED DESCRIPTION

Many known spool valves are controlled by an input force that urges aspool within the valve in one direction and that is counteracted by abiasing force to bias the spool in the opposite direction. In such knownspool valves the biasing force is generally fixed such that by varyingthe input force, relative to the biasing force, the position of thespool within the spool valve can be precisely controlled. In many knownspool valves, the input force is produced by an input pressure appliedto an end of the spool where the input pressure corresponds to aproportion of a supply pressure of a fluid being directed through thespool valve. Such spool valves are used in many different applicationsassociated with a wide range of supply pressures. Accordingly, the rangeof the corresponding input pressures of such spool valves also varieswidely between different applications.

For spool valves to function properly, the spool valve employs aconstant biasing force of suitable strength relative to the input forceto keep operating range of the spool valve centered within the operatingrange of the input force. Therefore, the biasing force needed to operatea spool valve in any particular application depends upon the supplypressure used in the application.

Unfortunately, many known spool valves have a fixed biasing forcedesigned for a particular supply pressure and/or narrow range of supplypressures. Accordingly, to satisfy the demand for spool valves to beused in different applications associated with wide ranging pressures,manufacturers are faced with the cost of producing and maintaining aninventory of multiple spool valves rated for the broad range of expectedsupply pressure. While the availability of such options allows end usersto acquire an appropriate spool valve for their desired applications,there is cost and complexity to end users in identifying and acquiringthe proper spool valve for their desired applications and/or when theyuse a spool valve in a new and different application.

To overcome these disadvantages, the examples disclosed herein includemeans for generating a spool biasing force based on a biasing pressurethat corresponds to the supply pressure. In this manner, as the supplypressure changes based on the particular application within which thespool valve is being implemented, the biasing pressure (and associatedbiasing force) changes proportionally. In this manner, manufacturers donot need to supply so many variations of a spool valve because a singlespool valve constructed in accordance with the teachings disclosedherein can be used in multiple applications associated with a broadrange of supply pressures. Furthermore, the example spool valvesdisclosed herein save end users cost in acquiring multiple spool valvesand reduce the complexity and risk of error in selecting a properlyrated spool valve for a particular application.

To achieve the proper balance of forces on a spool so that an increaseor decrease of the input force along the operational range of the inputforce, for a given supply pressure, results in a desired movement of thespool along the travel span of the spool, the biasing force at the givensupply pressure needs to fall within the operational range of the inputforce. That is, while the input force can vary between a minimum forceassociated with minimum input pressure (e.g., atmospheric pressure) anda maximum force associated with a maximum input pressure (e.g., the fullsupply pressure), the biasing force generated by the biasing pressuremust fall between the minimum and maximum force.

Establishing the maximum input force to be greater than the biasingforce, when both forces are based on the supply pressure, may beaccomplished in different ways. In some examples, the biasing force iscontrolled by reducing the area upon which the biasing pressure isapplied relative to the area upon which the input pressure is applied.For instance, in some disclosed examples, the biasing pressure isapplied to any of a diaphragm, a piston, or a bellows that defines anarea smaller than the area of the end of the spool to which the inputpressure is directly applied. In other examples disclosed herein, thebiasing force is controlled by reducing the biasing pressure relative tothe supply pressure. For instance, in some disclosed examples, thesupply pressure is passed through a first flow restrictor and partiallydirected out through a second flow restrictor leading to a lowerpressure (e.g., the atmosphere). In such examples, the first and secondflow restrictors are placed in series to define an intermediate pressurebetween the flow restrictors corresponding to the biasing pressure.

FIGS. 1-3 are cut-away perspective views of an example spool valve 100constructed in accordance with the teachings disclosed herein. Likeother spool valves, the example spool valve 100 includes a spool 102that is selectively moveable or slideable along a channel 104 of ahousing 106. In the illustrated example, the spool 102 is shown in afirst position corresponding to a minimum travel position in FIG. 1, ina second position corresponding to a null position in FIG. 2, and in athird position corresponding to a maximum travel position in FIG. 3. Insome examples, the spool 102 is throttled between the minimum travelposition and the maximum travel position based on an input force appliedto a first end 107 of the spool 102 relative to a biasing force appliedopposite the input force at a second end 108 opposite the first end 107.The comparative strength of the input force to the biasing forcedetermines whether the spool 102 moves and in which direction along itstravel span.

Near the mid-point and each of the ends 107, 108 of the spool 102 areportions where the spool 102 has a diameter larger than the rest of thespool 102 (referred to herein as lands 109) and which are dimensioned tosubstantially sealingly engage a sleeve 110 (e.g., via a sealing ringand/or via tight tolerances) disposed within the channel 104. Betweenthe lands 109 of the spool 102 there are portions having a smallerdiameter to define channels or grooves 112 that provide a path for fluidto travel between the lands 109 inside the sleeve 110. Further, as shownin the illustrated examples, the sleeve 110 has a plurality of openings113 in alignment with a plurality of ports 114, 116, 118, 120, 122 inthe housing 106 of the example spool valve 100 such that the channel 104(e.g., within the grooves 112 of the spool 102) is in fluidcommunication with the outside of the example spool valve 100. Further,in this manner, depending upon the position of the spool 102, one ormore of the ports 114, 116, 118, 120, 122 may be in fluid communicationto define a path of fluid to travel via one of the ports 114, 116, 118,120, 122, through the grooves of the spool 102, and out another one ofthe ports 114, 116, 118, 120, 122.

For example, when the spool 102 is in the minimum travel position asillustrated in FIG. 1, the locations of the lands 109 and the grooves112 define a first fluid path (indicated by arrow 124) between the ports114, 116 and a second fluid path (indicated by arrow 126) between ports118, 120. When the spool 102 is in the maximum travel position asillustrated in FIG. 3, the locations of the lands 109 and the grooves112 define a third fluid path (indicated by arrow 128) between the ports116, 118 and a fourth fluid path (indicated by arrow 130) between ports120, 122. When the spool 102 is in the null position as illustrated inFIG. 2, the lands 109 cover or block the openings 113 in the sleeve 110corresponding to the middle port 118 and the two outside ports 114, 122such that none of the ports 114, 116, 118, 120, 122 are in fluidcommunication.

The arrangement in the illustrated examples of FIGS. 1-3 is suitable forcontrolling a double-acting control valve as shown and described ingreater detail below in connection with FIG. 10. In particular, in someexamples, the spool valve 100 is configured such that the middle port118 receives a supply flow of pressurized fluid (e.g., pressurized air)such that the supply flow may be directed out either of the adjacentports 116, 118 depending upon the position of the spool 102 so long asthe spool 102 is not in the null position blocking the port 118. Thatis, as the spool 102 moves from the null position (FIG. 2) towards theminimum travel position (FIG. 1), the supply flow follows the secondfluid path in the direction indicated by the arrow 126 resulting in anoutput flow at the port 120 (e.g., the output B pressure shown in FIG.10). When the spool 102 moves from the null position (FIG. 2) toward themaximum travel position (FIG. 3), the supply flow follows the thirdfluid path in the direction indicated by the arrow 128 resulting in anoutput flow at the port 116 (e.g., the output A pressure shown in FIG.10). In such examples, as the actuator is actuated by the output flowfrom the port 116, displaced fluid within the actuator is forced backthrough the port 120 and then exhausted through the adjacent port 122(e.g., the exhaust B shown in FIG. 10) as the exhaust follows the fourthfluid path in the direction defined by the arrow 130. Likewise, when theoutput flow is from the port 120, displaced fluid within the actuator isforced back through the port 116 and then exhausted through the adjacentport 114 (e.g., the exhaust A shown in FIG. 10) as the fluid follows thefirst fluid path in the direction defined by the arrow 124.

As described above, in some examples, the movement of the spool 102 iscontrolled by an input force applied to the first end 107 of the spool102 relative to a biasing force applied opposite the input force to thesecond end 108 of the spool 102. In some examples, the input force isgenerated from an input pressure applied to the first end 107 of thespool 102. In some examples, the input pressure is provided via a supplypressure that is separately coupled to a supply port of the spool 102(e.g., the middle port 118 as described above). More particularly, insome examples, the input pressure corresponds to a proportional amountof the supply pressure defined by an electrical input signal generatedas part of a control strategy in a process control system (e.g., a 4-20milliamp (mA) proportional signal). That is, in some examples, the inputpressure has an operational range between zero or nearly zero pressure(e.g., atmospheric pressure) and the pressure of the supply asdetermined from the input signal provided to a current-to-pressure (I/P)converter (e.g., the I/P converter 1008 of FIG. 10).

In many known spool valves, the biasing force to counteract the inputforce is provided by a control spring within the spool valve. Thecontrol spring has a predetermined initial compression to act upon theopposite end of the spool. In such known spool valves, as the inputforce increases (e.g., based on an increase in the input pressure) thespool moves towards the control spring, thereby compressing the springand increasing the biasing force until the spool stops moving when theinput force and the biasing force are approximately equal. As the inputforce decreases, the control spring pushes the spool back towards theinput end of the spool. Accordingly, many known spool valves require acontrol spring of suitable strength based on the operational range ofthe input pressure. That is, if the control spring is not strong enoughbecause of a high input pressure range (e.g., due to a high supplypressure), the force from the input pressure overcomes the controlspring and prevents the control spring from proper biasing the spool.Similarly, if the control spring is too strong because of a low inputpressure range, the force from the input pressure is unable to move thespool as desired. Accordingly, the ratings of the control springs usedin many known spool valves are application specific in that the springsmust be selected based on the supply pressure (and corresponding rangefor the input pressure). As a result, if end users desire to implement aspool valve in a different application with a different supply pressure,they must determine and acquire an appropriately rated spring for thenew application and then disassemble and exchange the springs beforeusing the spool valve. Alternatively, end users need to have acompletely separate spool valve that can handle the pressures associatedwith the application of interest. Either option presents costs,complexity, and inconvenience to the implementation of such spool valvesin multiple different applications.

The examples disclosed herein overcome these disadvantages of knownspool valves by generating the biasing force from a biasing pressureapplied to the second end 108 of the spool 102, where the biasingpressure is based on the supply pressure. In this manner, the biasingforce increases or decreases proportionally to any increase or decreasein the operational range of the input force because both the input forceand the biasing force are proportional to the supply pressure. As theinput pressure corresponds to a proportion of the supply pressure (basedon a proportional input signal), the maximum input force on the spool102 corresponds to the input pressure equaling the supply pressure. Assuch, directly applying the supply pressure to the opposite end of thespool 102 (e.g., the biasing pressure is the same as the supplypressure) results in the biasing force being equivalent to the maximuminput force. As a result, any lesser input force results in the biasingforce overcoming the input force, thereby preventing proper control ofthe position and/or movement of the spool 102. Accordingly, in someexamples, the spool valve 100 is constructed such that the biasingforce, although based on the supply pressure, is less than the maximuminput force. In some examples, establishing a biasing force that is lessthan the maximum input force is accomplished by using the supplypressure as the biasing pressure but reducing the area of the spool 102on which the biasing pressure is applied. In some examples, the biasingpressure applied to the spool 102 is controlled such that it is lessthan supply pressure (e.g., the biasing pressure is a proportion of thesupply pressure), thereby reducing the biasing force even if the area onwhich the biasing pressure is applied is the same as the area on whichthe input pressure applies on the input side of the spool 102.Additionally or alternatively, in some examples, both the biasingpressure relative to the supply pressure and the area on which thebiasing pressure is applied relative to the area on which the inputpressure is applied may be varied in any suitable manner to establishthe desired relationship between the input force and the biasing force.

In the illustrated examples of FIGS. 1-3, the example spool valve 100includes a diaphragm 132 with an area that is smaller than the area ofthe input end of the spool 102. In this manner, when the input pressureis equivalent to or approaching the full supply pressure the resultinginput force will be greater than the biasing force because the inputpressure will be acting on a larger area than the area of the diaphragm132 on which the biasing pressure (i.e., the supply pressure) acts. Insome examples, the diaphragm 132 is coupled to the spool 102 via apiston 134 having a cross-sectional area approximately the same as thearea of the diaphragm 132. As the diameter of the piston 134 is smallerthan the outer diameter of the spool 102 (corresponding to the smallerdiameter of the diaphragm 132), in some examples, the spool valve 100includes a spacer 136 to surround the piston 134 and hold the piston 134in place.

The area of the diaphragm 132 upon which the biasing pressure is appliedto generate the biasing force is based on the operational range of theinput pressure as dictated by the I/P converter and the correspondingcontrol strategy. In some examples, the diaphragm 132 has asubstantially fixed area regardless of the position of the spool 102along its travel span. In such examples, although the area issubstantially constant and the applied biasing pressure is substantiallyconstant (e.g., the supply pressure is substantially constant), thebiasing force nevertheless varies across the travel span of the spool102 because of the resilience of the diaphragm 132. In this manner, achange in the input pressure causes the spool 102 to move untilequilibrium between the input force and the biasing force is achieved,thereby allowing for precise control of the position of the spool 102similar to the control springs of known spool valves described above.Additionally or alternatively, in some examples, the spool valve 100includes a biasing spring 138 to augment the change in the biasing forcealong the travel span of the spool 102 as the biasing spring 138elongates and/or compresses. In some examples, where the biasing spring138 is not used to counteract the input force, the biasing spring 138 isnevertheless included within the example spool valve 100 to provide afailsafe to bias the spool 102 to a failure position if the supplypressure and corresponding input pressure and biasing pressure are lost(e.g., substantially reduced or zero). In some such examples, thebiasing spring 138 may have a substantially lower spring rate thancontrol springs used in known spool valves described above because thebiasing spring 138 does not have to counteract the force of the inputpressure on the spool 102.

In some examples, the diaphragm 132 is omitted and the biasing pressureis applied directly to the piston 134, which defines the same fixedarea, to produce the biasing force. In some such examples, the piston134 is fabricated to fit within the spacer 136 within tight tolerancesto reduce (e.g., minimize) leakage. Additionally or alternatively, insome examples, leaks are reduced via a sealing ring placed between thepiston 134 and the spacer 136.

In other examples, a bellows is used instead of a diaphragm to define areduced area upon which the biasing pressure may be applied to generatethe biasing force as shown in the example spool valve 800 of FIG. 8. Theexample spool valve 800 may be constructed from components of the sametype of spool valve as the example spool valve 100 of FIGS. 1-3.Accordingly, the example spool valve 800 includes the spool 102, thesleeve 110, and the housing 106. However, unlike the example spool valve100 of FIGS. 1-3, the example spool valve 800 of FIG. 8 includes abellows 802 within a bellows housing 804. In some examples, the bellows802 is coupled to the spool 102 via an adapter 806, which is held inalignment with the spool 102 via a spacer 807. In the illustratedexample, the bellows 802 is enclosed at the opposite end by an end cap808. As shown in the illustrated example, the bellows 802 defines areduced area 810 upon which the biasing pressure (e.g., the supplypressure) is applied such that the resulting biasing force is less thanthe maximum input force based on the supply pressure applied to theopposite end of the spool 102. In some examples, the bellows 802,adapter 806, and end cap are rigidly connected. In this manner, as thespool 102 is moved within the sleeve 110, the bellows 802 willcorrespondingly expand or contract in the direction of the movement ofthe spool 102. In some examples, the bellows 802 also serves as thefailsafe to urge the spool 102 toward a desired failure position if thesupply pressure is lost (e.g., substantially reduced or zero).

FIGS. 4 and 5 are respective cross-sectional and exploded views of theexample spool valve 100 of FIG. 1. As shown in the illustrated example,the spool 102 is disposed within the sleeve 110. The piston 134 issurrounded by the spacer 136 and operatively couples the spool 102 tothe diaphragm 132, which is held in a diaphragm housing 140. A springbarrel 142 is attached to the diaphragm housing 140 to secure a springseat 144 for the biasing spring 138. A supply side end cap 146compresses the spring within the spring barrel 142 to apply apredetermined amount of force to the spool 102 to bias the spool to afailsafe position if there is a failure in the supply of pressure. Insome examples, the spool valve 100 is sealed via an o-ring 148 betweenthe supply side end cap 146 and the spring barrel 142. Further, as shownin the illustrated example of FIGS. 4 and 5, at the opposite end of theexample spool valve 100, the end of the spool 102 is surrounded byadditional spacers 150 and enclosed by an input side end cap 152.

In some examples, the spool valves described herein are manufacturedusing components of an existing spool valve in combination with newcomponents constructed in accordance with the teachings disclosedherein. In this manner, existing spool valves may be modified toimplement the teachings disclosed herein. For instance, the examplespool valve 100 of FIGS. 1-5 may be constructed by using some of thecomponents of a NUMATICS® 2035 valve made by Numatics Inc., of Novi,Mich. and replacing the other components. In particular, the examplespool valve 100 in the illustrated examples corresponds to a NUMATICS®2035 valve that has been modified in that the original spring seat,control spring, and end cap have been replaced with the piston 134, thespacer 136, the diaphragm 132, the diaphragm housing 140, the springseat 144, the biasing spring 138, the spring barrel 142, and the supplyside end cap 146.

FIGS. 6 and 7 are respective cross-sectional view and exploded views ofanother example spool valve 600 constructed in accordance with theteachings disclosed herein. The example spool valve 600 may beconstructed from components of a NUMATICS® PA 15 valve made by NumaticsInc. in a manner similar to that noted above in connection with theexample spool valve 100 of FIGS. 1-5. In particular, the example spoolvalve 600 includes a spool 602 moveable along a sleeve 604 disposedwithin a housing 606, all of which correspond to original components ofa NUMATICS® PA 15 valve. However, the example spool valve 600 has beenprovided with new components constructed in accordance with theteachings disclosed herein including a piston 608, a spacer 610, adiaphragm housing 612, a diaphragm 614, a spring barrel 616, a springseat 618, a biasing spring 620, and a supply side end cap 622. In someexamples, an input pressure, which may be proportional to a supplypressure, is applied to a first end 624 of the spool 602 to generate aninput force on the spool 602. Further, in some examples, a biasingpressure is applied to the diaphragm 614 to generate a biasing force onthe spool 602 via the piston 608 at a second end 626 of the spool 602opposite the first end 624. In some examples, the supply pressure servesas the biasing pressure. In such examples, the area of the diaphragm 614is designed based on the operational range of the input force to enablecontrol of the movement of the spool 602 within the sleeve 604 based ondifferences in the resulting input and biasing forces regardless of theamount of supply pressure. In this manner, the supply pressure serves asthe basis for both the input pressure and the biasing pressure asdescribed above.

FIG. 9 is a cross-sectional view of another example spool valve 900constructed in accordance with the teachings disclosed herein. Theexample spool valve 900 may be constructed from components of aNUMATICS® PA 15 valve in a manner similar to that noted above inconnection with the example spool valve 600 of FIGS. 6 and 7.Accordingly, the example spool valve 900 includes the spool 602, thesleeve 604, and the housing 606. However, unlike the example spool valve600 of FIGS. 6 and 7, the example spool valve 900 of FIG. 9 includes aspring seat 902 that is coupled directly to the spool 602. In someexamples, the spring seat 902 is the original spring seat manufacturedwith the NUMATICS® PA 15 valve on which the example spool valve 900 isbased. In the illustrated example, the original components of theNUMATICS® PA 15 valve have been combined with new components constructedin accordance with the teachings disclosed herein. In particular, theexample spool valve 900 includes a spring barrel 904 that defines achamber 906 enclosed by an end cap 908. In the illustrated example, thechamber 906 houses a biasing spring 910 to act on the spring seat 902 inthe same manner as described above for the biasing spring 138 of theexample spool valve 100 of FIGS. 1-5.

As shown in the illustrated example of FIG. 9, the spring barrel 904includes a first flow restrictor 912 and a second flow restrictor 914fluidly coupled in series with the chamber 906. In some examples, thefirst flow restrictor 912 is coupled to the supply pressure for thespool valve 900 such that the supply pressure is in fluid communicationwith the chamber 906. Further, in such examples, the second flowrestrictor 914 is exposed to a second pressure lower than the supplypressure such that the chamber 906 is in fluid communication with thesecond pressure. In some examples, when the fluid is pressurized air,the second flow restrictor leads from the chamber 906 to the atmosphereoutside the spool valve 900 (i.e., the second pressure is atmosphericpressure). In this manner, as the supply fluid (e.g., air) fills thechamber 906, some of the pressure within the chamber 906 bleeds out tothe atmosphere resulting in an intermediate pressure within the chamber906 that is directly applied to the second end 626 of the spool 602(i.e., the biasing pressure) to bias the spool 602 against the inputpressure applied to the first end 624 of the spool 602. In suchexamples, unlike the example spool valves 100, 600, 800 of FIGS. 1-8,the area on which the biasing pressure is applied in the example spoolvalve 900 of FIG. 9 is not defined to be smaller than the area on whichthe input pressure is applied to lower the resulting biasing forcerelative to the maximum input force. Rather, the example spool valve 900is configured such that the intermediate or biasing pressure is betweenthe atmospheric pressure and the supply pressure. That is, in theillustrated example of FIG. 9, the full supply pressure is not appliedto spool 602 but an intermediate pressure (e.g., the biasing pressure)is applied as a result of the constant bleeding of pressure out to theatmosphere via the second flow restrictor 914. In some examples, theprecise dimensions of the first and second flow restrictors 912, 914 arebased on the operational range of the input pressure such that theresulting input force and biasing force (based on the intermediatepressure) enable control of the spool 602 along its travel span.Accordingly, in some examples, the intermediate pressure will increaseor decrease proportionally with any increase or decrease in the supplypressure resulting in a corresponding increase or decrease in thebiasing force to counteract the input pressure in a similar manner tothe reduced area on which the biasing pressure acts in the example spoolvalves 100, 600, 800 of FIGS. 1-8 described above.

As described above, in some examples, the biasing spring 910 serves as afailsafe to bias the spool 602 to a desired failure position if thesupply pressure (and corresponding input pressure and biasing pressure)is lost (e.g., substantially reduced or zero). Additionally oralternatively, in some examples, the biasing spring 910 also serves topartially bias the spool 602 along with the biasing force from thebiasing pressure to create a variation in the bias force correspondingto the position of the spool 602 (e.g., based on the elongation and/orcompression of the biasing spring 910).

Although the example spool valves 100, 600, 800, 900 have been describedin detail above, the teachings disclosed herein are also applicable toother spool valves. For example, other spool valves having more or fewerports defining more, fewer, and/or different fluid communication pathsbetween the ports can be modified in accordance with the teachingsdisclosed herein to bias the corresponding spool based on a supplypressure. In some such examples, the resulting biasing force may bedefined by designing an appropriate area (e.g., corresponding to thearea of a diaphragm, piston, or bellows) on which the supply pressure isapplied. In other examples, a biasing pressure corresponding to aproportion of the supply pressure is applied to the spool to achieve thedesired biasing force. Additionally or alternatively, in some examplesboth the biasing pressure and the area on which the pressure is appliedcan be specifically designed in accordance with the teachings disclosedherein to establish the proper relationship between the input force andthe resulting biasing force that applies over a broad range of potentialsupply pressures because both the input pressure and biasing pressureare based on the supply pressure. Further, although the example spoolvalves 100, 600, 800, 900 described above are pneumatic spool valves,the teachings disclosed herein may also be suitably adapted to hydraulicspool valves. Additionally, the example spool valves 100, 600, 800, 900disclosed herein, as well as other spool valves constructed inaccordance with the teachings disclosed herein, may be implemented inany suitable application for such spool valves. For instance, asmentioned above, the example spool valves 100, 600, 800, 900 may be usedto control the position of a control valve as shown and described morefully in FIG. 10. Furthermore, the teachings disclosed herein can alsobe used to bias a spool valve based on supply pressure when the inputforce is not based on the supply pressure (e.g., solenoid actuated spoolvalves, manually actuated spool valves, etc.)

FIG. 10 is a schematic drawing of an example digital valve controller(DVC) 1000 that includes the example spool valve 100 of FIGS. 1-5. Theexample DVC 1000, as with other known DVCs, includes a printed wiringboard 1002 configured to control a double-acting control valve 1004based on an electrical input signal (e.g., from a control room of aprocess control system) and feedback from a position sensor 1006associated with the control valve 1004. In the illustrated example, theDVC 1000 receives a supply pressure that is directed through acurrent-to-pressure (I/P) converter 1008 to provide an input pressureproportional to a drive signal generated by the printed wiring board1002 based on the input signal and the position feedback.

In many known DVCs, the input pressure is provided to an internal relaythat uses the input pressure to form a proportional pressure (or flow)via one of two outputs (e.g., output A and output B) that are coupled tothe control valve 1004 to precisely control movement of the valve.However, such known DVCs are limited because the internal relay cannothandle high flow rates (e.g., high supply pressures). As a result, suchDVCs are limited to controlling actuators with smaller volumes and/ormoving control valves at slower speeds. To overcome these disadvantages,some known DVCs are coupled to a pneumatic volume booster to achievehigher pressures/flow rates. However, such a solution is expensive as itrequires the additional component of the pneumatic booster and theresulting expense of maintaining additional components. Furthermore,volume boosters can be difficult to adjust or change (e.g., when usingthe DVC in a different application).

In accordance with the teachings disclosed herein, the example DVC 1000includes the spool valve 100 instead of an internal relay because spoolvalves can handle significantly higher pressure ranges than knownrelays. The other spool valves 600, 900 described herein or anotherspool valve constructed in accordance with the teachings disclosedherein could alternatively be used in place of the spool valve 100. Asshown in the illustrated example, the supply pressure is directed to theinput side of the example spool valve 100 (e.g., via the I/P converter1008), to the supply port of the spool valve 100 (e.g., the middle port118), and to the supply side of the example spool valve 100. In thismanner, the input pressure, which is based on the supply pressure on theinput side of the example spool valve 100, generates an input force onthe spool 102 that is counteracted by a biasing force on the spool 102.The biasing force is generated by the biasing pressure, which is alsobased on the supply pressure but on the supply side of the example spoolvalve 100. As both the input pressure and the biasing pressure are basedon the supply pressure, the resulting input force and biasing force areproportional to each other. In this manner, the example DVC 1000 canhandle a broad range of supply pressures (e.g., between 20-150 psi)without an operator or other personnel having to adjust a pneumaticvolume booster (if a relay is used) and/or keep track of or interchangemultiple control springs (if a known spool valve is used) when thesupply pressure is changed. In such examples, the relationship of thebiasing force to the operational range of the input force is controlledby designing the size of the area on which the biasing force is applied(e.g., when the biasing pressure is the same as the supply pressure) tobe smaller than the area on which the input pressure applies and/or bydesigning the spool valve 100 to control the biasing pressure to belower than the supply pressure.

Although certain example methods, apparatus and articles of manufacturehave been disclosed herein, the scope of coverage of this patent is notlimited thereto. On the contrary, this patent covers all methods,apparatus and articles of manufacture fairly falling within the scope ofthe claims of this patent.

What is claimed is:
 1. An apparatus comprising: a housing of a spoolvalve, the housing including a first port to receive a fluid at a supplypressure; and a spool within the housing, a position of the spool to beselectively controlled via an input force acting on the spool, theposition of the spool to define a path of a flow of the fluid throughthe spool valve from the first port to a second port of the housing,wherein a biasing force is to bias the spool opposite the input force,the biasing force to be generated from the supply pressure applied to anend of the spool.
 2. The apparatus of claim 1, further comprising apiston coupled to the end of the spool, the biasing force to bias thespool via the piston.
 3. The apparatus of claim 1, further comprising abellows coupled to the end of the spool, the biasing force to bias thespool via the bellows.
 4. The apparatus of claim 1, further comprising adiaphragm coupled to the end of the spool, the biasing force to bias thespool via the diaphragm.
 5. The apparatus of claim 1, further comprisinga spring to bias the spool to a failure position if the supply pressureis substantially reduced or zero.
 6. The apparatus of claim 1, whereinthe input force is generated from an input pressure applied to a secondend of the spool, the input pressure corresponding to a proportion ofthe supply pressure.
 7. The apparatus of claim 6, wherein the supplypressure is to be applied to a first area adjacent the first end of thespool and the input pressure is to be applied to a second area adjacentthe second end of the spool, the first area smaller than the secondarea.
 8. The apparatus of claim 7, wherein the first area is defined byone of a bellows, a piston, or a diaphragm.
 9. The apparatus of claim 1,further comprising first and second flow restrictors fluidly coupled inseries via a chamber defined by the housing, the supply pressure toenter the chamber via the first flow restrictor, a portion of the supplypressure to escape the chamber via the second flow restrictor to definean intermediate pressure within the chamber, the biasing force to begenerated by the intermediate pressure applied to the end of the spool.10. An apparatus comprising: a spool within a spool valve to control aflow of fluid at a supply pressure through the spool valve as the spoolis moved via an input force, the spool valve to define a chamberadjacent an end of the spool, wherein a biasing force is to bias thespool opposite the input force, the biasing force to be generated from abiasing pressure of the fluid within the chamber, the biasing pressureto be based on the supply pressure.
 11. The apparatus of claim 10,further comprising: a first flow restrictor to provide fluidcommunication between the chamber and the fluid at the supply pressure;and a second flow restrictor to provide fluid communication between thechamber and a second pressure lower than the supply pressure, the firstand second flow restrictors disposed in series on the spool valve withthe chamber disposed therebetween, wherein the first and second flowrestrictors define the biasing pressure within the chamber, the biasingpressure to be between the second pressure and the supply pressure. 12.The apparatus of claim 10, wherein the input force is generated from aninput pressure applied to a second end of the spool, the input pressureto be a proportion of the supply pressure ranging from a minimumpressure to a maximum pressure corresponding to the supply pressure. 13.The apparatus of claim 12, wherein the biasing pressure is less than themaximum pressure.
 14. The apparatus of claim 12, further comprising acurrent-to-pressure converter of a digital valve controller to definethe proportion of the supply pressure based on an electrical controlsignal.
 15. The apparatus of claim 10, further comprising a digitalvalve controller, wherein the spool valve is to be housed in the digitalvalve controller.
 16. The apparatus of claim 10, wherein the supplypressure is to be applied to a first area within the chamber adjacentthe end of the spool, the input force to be generated from an inputpressure applied to a second area adjacent an opposite end of the spool,the input pressure corresponding to a proportion of the supply pressure,the first area smaller than the second area.
 17. An apparatuscomprising: a spool within a spool valve, the spool to be selectivelymoveable within the spool valve via an input force to control a flow ofa supply fluid between ports in the spool valve, the supply fluid havinga supply pressure; and means for generating a biasing force to bias thespool opposite the input force, the biasing force corresponding to abiasing pressure applied to an end of the spool, the biasing pressure tobe based on the supply pressure of the supply fluid.
 18. The apparatusof claim 17, wherein the means for generating the biasing force includesmeans for reducing a first area on which the biasing pressure is appliedrelative to a second area on which an input pressure is applied, theinput pressure to define the input force.
 19. The apparatus of claim 17,wherein the means for generating the biasing force includes means forreducing the biasing pressure relative to the supply pressure.
 20. Theapparatus of claim 16, wherein a change in the supply pressure is toresult in a proportional change in the biasing force.